1. Field of the Invention
The present invention relates to a signal detection apparatus that can accurately detect signals whose frequency is unknown or changes over time.
2. Description of Related Art
Conventionally, distributed power sources are known as systems that supply power to electric systems. Ordinarily, in distributed power sources, the phase of an AC voltage signal or an AC current signal of the electric system (“system signal”) is detected, and this phase is fed back to the distributed power source. Thus, a control can be performed, by which the phase of the AC signal that is output from the distributed power source is matched to the phase of the system signal.
Since it is required that the distributed power source matches the phase of the output signal quickly and with high precision to the phase of the system signal, several circuits have been conventionally suggested for detecting the system signal. For example, in JP2000-116148A, a signal detection circuit is proposed, in which a phase voltage signal of a three-phase electric system is detected, this phase voltage signal is converted into a phase voltage signal of the positive phase part of each phase by a symmetric coordinate conversion circuit, and after normalization with a normalization circuit, a voltage signal is generated and output by a PLL (Phase-Locked Loop) circuit, that is synchronized only with the positive phase part of the phase voltage signal.
The signal detection method of JP 2000-116148A is referred to as “PLL method”. The PLL method (1) generates an AC signal from a signal generator with variable phase, feeds back this AC signal to the input signal and calculates the phase shift between the two signals as a control signal, (2) synchronizes the phase of the AC signal to the input signal with this control signal by changing the phase of the AC signal generated by the signal generator in a direction that reduces the phase shift, thereby outputting a signal that is the same as the input signal. Consequently, since a detection circuit using this PLL method requires a signal generator and a circuit for the feedback control of the phase of the output signal of this signal generator (a circuit such as a phase comparator or a loop filter), the circuit configuration becomes complicated. Also, in the phase detection circuit described in JP 2000-116148A, there is no configuration for eliminating the noise components that are included in the input signal, so that if the frequency of the input signal is unknown or changes over time, there is the problem that the input signal cannot be detected accurately.